1. Field of the Invention
This invention relates to low viscosity, reactive silicone sealant compositions for sealing porosity in plastics, metals and other porous materials requiring a fill and/or seal. More particularly, the invention relates to silicone sealant compositions having high thermal and chemical resistance.
2. Description of Related Art
Impregnation sealing of microporosity is a commonly used methodology in the art of forming a variety of articles, structural components, and assemblies, as for example castings, die castings, electronic components, powder metal parts, fiber-reinforced resin composites and other materials which exhibit porosity.
Originally, materials, manufacturing techniques, and casting designs were specified to minimize the occurrence of porosity in formed objects, based on the hypotheses that microporosity was structurally and functionally undesirable and its presence in formed articles embodied poor manufacturing. This approach severely limited design freedom, and resulted in significant rejection of parts exhibiting any substantial porosity characteristics.
This design strategy changed in the 1970s as a result of the energy crisis, which resulted in a major switch to lighter metals for structural applications. During this period many iron parts were changed to cast aluminum components, and many other parts were designed as die castings. This switch to lighter metals resulted in weight savings in many applications where energy consumption and power optimization were important, but created a new and persistent problem of microporosity in the light metal formed parts. The occurrence of microporosity is particularly acute in components formed from metal powder, and presents a significant obstacle to commercial utility, particularly when such porous parts are employed in fluid power systems and other liquid handling applications.
In order to overcome the deficiencies attendant the presence of microporosity in formed articles of the above-described types, impregnation sealing technology was developed, by which the porosity of the porous parts was impregnated with a sealant composition. Upon curing of the impregnated sealant, the resulting sealed part is suitable for use in fluid exposure applications, as well as facilitating plating, coating, and further processing of the formed article.
Among the impregnation sealing compositions which have been developed to date are self-curing anaerobic sealants and thermal curing sealants, as well as sealants which cure by both anaerobic and heat cure mechanisms.
Electronic encapsulating sealant/coating compositions, curable both anaerobically and with exposure to UV light, have also been developed for vacuum impregnation of electrical components such as transformers, wherein the encapsulating sealant is anaerobically cured inside the device and is cured on the outside surface with UV light to encapsulate the device. To effect a thorough outer surface curing of the sealant, such compositions typically contain a UV photoinitiator in concentrations substantially in excess of 5% by weight based on the weight of the curable component thereof.
In addition, sealant/coating compositions have been developed for sealing of laminates, composite materials, and the like, containing macroscopic or gross voids into which the sealant/coating composition after surface application flows by capillary, or wicking action. Generally, sealant/coating compositions employed in such applications are highly viscous in character, having a viscosity substantially greater than 1000 centipoise, as measured by the Cannon-Fenske viscosity determination method. One such conventional formulation, having a Cannon-Fenske viscosity of 4200 centipoise, contains 3.4 weight percent of a UV photoinitiator, based on the weight of curable component in the sealant/coating composition, to effect surface cure of the composition under UV radiation, in combination with internal anaerobic curing of the composition. The high viscosities of such compositions generally require long processing times for impregnation of microporosity.
Among the previously developed impregnating compositions for sealing porous parts are the compositions disclosed in the patents identified and discussed below.
U.S. Pat. No. 3,672,942 to Neumann discloses an anaerobic impregnant comprising a free-radical polymerizable acrylate ester monomer and free-radical polymerization initiator therefor, e.g., a hydroperoxide. The patent discloses utilizing an accelerator in the impregnant, such as aldehyde-amine condensation products, sulfur-containing free-radical accelerators, or organic compounds containing an oxidizable transition metal. This reference also discloses a vacuum impregnation process in which the porous article is placed in a vacuum vessel, followed by drawing of vacuum therein and covering the article with the disclosed anaerobic sealant so that upon release of vacuum, the sealant is forced into the evacuated porosity of the article. The surface of the impregnated article then is treated with the aforementioned polymerization accelerator to cure the sealant at the outer surface of the porous article.
U.S. Pat. No. 3,969,552 describes an impregnation composition comprising an acrylic anaerobic curing resin and a peroxy initiator therefor. The wash solution is an aqueous solution of a surfactant of specified formula. The patent further discloses that the aqueous surfactant solution may contain an accelerator to effect polymerization of the anaerobic sealant in the surface areas of the impregnated part being washed.
U.S. Pat. No. Re. 32,240 to DeMarco describes a self-emulsifying anaerobic composition for porosity impregnation applications, comprising an anaerobically curing monomer such as an acrylic ester, a peroxy initiator therefor, e.g., a hydroperoxide or perester, an anionic or nonionic surfactant which is dissolved in the composition and renders it self-emulsifying upon mixing with water, and optionally an accelerator for the anaerobic polymerization, e.g., a sulfimide.
U.S. Pat. No. 4,632,945 to Garcia, et al, discloses an anaerobic sealant material comprising a (meth)acrylate monomer, a hydroperoxide or perester initiator, an accelerator having --SO.sub.2 NCO-- functionality, and a transition metal co-accelerator comprising a source of copper iron and an iron salt or ferrocenyl compound.
The above-described anaerobic sealant compositions are typically impregnated in the porosity of porous metal parts by wet vacuum impregnation, wet vacuum/pressure impregnation, or dry vacuum/pressure impregnation.
All of these and other known reactive sealant compositions are either polyester or organic (meth)acrylate monomer based. Hence as a consequence, presently available reactive sealant compositions are severely limited in that they have thermal resistance up to around 350.degree. F. After such temperature is reached the cured resin decomposes and thereby shrinks away from within the interior of the pores or reduces in weight thereby breaking the seal. Additionally, such prior art sealant compositions if not fully cured within the porosity of parts and components used in electronics, can be somewhat conductive and therefore interfere with electrical properties of the parts.
Hence, it would be a significant advance in the art to provide a sealant composition having improved thermal and chemical resistance and which is non-conductive, even when not fully cured thereby not interfering with electrical properties of impregnated electronic parts.